In the field of nuclear magnetic resonance (NMR) several types of examinations have been utilized to provide fast and accurate imaging. During a typical NMR examination a patient is subjected to a uniform, intense, continuous, magnetic field, which is sometimes referred to as an orienting field. Magnetization of molecular protons within a body of the patient, especially those contained within hydrogen atoms or water molecules, are oriented along an applied direction of the orienting field. The protons are then subjected to an excitation, in the form of radio frequency energy, which causes their magnetization to “flip”. As the magnetization of the protons return to the original direction of magnetization a precession signal is generated, measured, and processed. The precession signal essentially provides density information of the different materials of the body being examined. In general, the materials having a greater amount of hydrogen provide a stronger contribution to the precession or response signal.
Generally, all the parts of the body that are subjected to the excitation respond simultaneously. As such, the response signal is generated in response to all of the molecules within the body parts that return to equilibrium after excitation. To discriminate between the contributions provided by each of the body parts, it is necessary to encode the excitation signal and decode the response signal. Encoding includes reiteration of the excitation signal and thus generation of the response signal. As many different encoding and measurement operations are needed as there are pixels in an image to be reconstructed. The reiteration of the excitation and response signals increases time to acquire an image due to the duration of each excitation and response sequence.
Two fast sequencing techniques that are used to decrease imaging time are fast spin echo (FSE) and steady-state free precession (SSFP). Time available to perform the sequences of FSE, SSFP, and the like is limited. For example, the time to perform a couple of excitations and signal acquisitions can be reduced down to 3 ms. It is the advent of compensated gradient coils, which eliminate eddy current effects, that enables the acquisition of data at this fast rate.
Referring now to FIG. 1, a Cartesian coordinate system diagram 10 illustrating a transient response of magnetization from an initial magnetization M0 to a steady-state magnetization MSS for an application of SSFP is shown. The orienting field B0 is applied along the z-axis. Magnetization at rest and before excitation of the cells under examination, due to the orienting field B0, is represented by vector M0. After a certain number of RF excitation pulses, magnetization is oriented to the steady-state magnetization MSS. The steady-state magnetization MSS is at a tilt angle ψ from the magnetization at rest M0. The real-valued eigenvector vr is shown. The component of the transient response, which is directed along vr decays exponentially (and is not shown). The component that is orthogonal to vr decays along a circular spiral path, designated as wT, in the plane approximately orthogonal to vr.
The value of the tile angle ψ depends upon various factors, such as the relaxation times T1 and T2 of the body cells being examined. The time T1 is a time constant corresponding to the exponential recovery of the longitudinal magnetization component of the cells that are aligned with the magnetic field B0. The time T2 is a time constant corresponding to the exponential decrease or decay in the transverse magnetization component of the cells. After preparation of the steady-state magnetization MSS a large number of excitations and measurements can be performed.
The steady-state magnetization MSS, for the fast sequences, is not reached quickly from the application of RF pulses. The steady-state magnetization MSS is attained only after a large number of RF pulses have been applied, corresponding to a length of time that is approximately three or four times the duration of the time T1. The duration of the combined pulses is greater than the duration of a measurement. In other words, the steady-state magnetization MSS is not attained for a short duration measurement. As a result, the beginning of each excitation signal or burst is not at steady-state equilibrium and a long transient unstabilized signal exists during which no useful data can be acquired.
Also, since the direction of the steady-state magnetization vector MSS depends from T1 divided by T2 or the inverse, image generation using the stated technique is not particularly useful for the examination of a brain. The stated technique is useful in other tissue examinations, such as in cardiac type examinations. However, in the cardiac field, the stated fast sequences require an undesirable length of time to perform. The cyclical character motion of the heart correspondingly requires that the measurements performed be assigned to a particular time in the cardiac cycle and to a precise position in a section of the heart. For example, when sequences are performed for 30 sections of the heart they must be assigned to a cardiac phase among 15 possible phases. In so doing, acquisition must correspond to the measurement of 450 sections. In practice, even at high working speeds, this acquisition can take more than 2 min. Unfortunately, pulse stability of the heart is not acquired over such a long duration.
Two methods have been introduced in an attempt to stabilize material magnetization and decrease sequence performance time. However, these methods have been shown to be imperfect, complicated, and not robust. In the first method a series of six preparation pulses are utilized. The duration of each excitation pulse can be approximately 2 ms in duration. Little time remains for performing a measurement in a total limited time duration of 3 ms. Also, the preparation for the steady-state magnetization is highly sensitive to the calibration of the excitation, thus, rendering it not robust for industrial-scale applications.
In the second method, the steady-state magnetization is attained by applying a series of RF pulses that have amplitudes, which increase linearly and at a constant pitch from pulse to pulse. Unfortunately, although in theory the tilt angle ψ is attained at the end of the RF pulses, in reality an oscillation remains in the perpendicular component of the magnetization. While the oscillation remains accurate data cannot be obtained. Thus, one must wait until the oscillation dampens before acquiring the data. As a result of the oscillation, time to reach steady-state magnetization and imaging time remains longer than desired. The two methods and their associated disadvantages are described in greater detail below in the Detailed Description.
Thus, there exists a need for an improved NMR excitation method that allows for the performance of fast NMR sequences, which is accurate, simple, and robust, and provides decreased imaging time.